One of the most common mechanical fastening methods is the bolted joint. In this type of joint, a first object and a second object are pulled together by means of an externally-threaded bolt mated to an internally-threaded nut. Generally, the bolt passes through a hole in each of the two objects, with the inner face of the bolt head seating against the outer surface of one of the objects and the inner face of the nut seating against the outer face of the remaining object. Often, a washer will be placed under either one or both of the bolt head and/or nut in order to protect the surface of the objects.
The bolted joint has enjoyed a great deal of success as a fastening method, and has been employed in all manner of mechanical devices and structures. Where a bolted joint is anticipated as the preferred method of attachment, one or both of the objects to be secured together will be “counter-bored” or “spot-faced” on one or both of the faces mating with the bolt and nut so as to provide a pair of flat, parallel surfaces suitable for maximum mechanical integrity.
One rationale behind the counter-boring and spot-facing operations is that bolts are generally very strong in pure tension. As bending loads are imparted to a bolt, however, strength may be reduced. While pure tension loading imparts a uniform stress across the cross-sectional area of the bolt, bending load represents a concentration of tensile load on one side of the bolt, and a corresponding concentration of compressive loading on the opposite side. In severe cases of misalignment, a bolt may experience a sufficient bending load that mechanical failure occurs.
In order to prevent the impartation of bending load to mechanical fasteners, a number of methods have been devised for creating and maintaining a parallel geometry. These methods include the introduction of shims in between the fastener head and a non-parallel surface in order to create a parallel geometry. Each of these methods has a number of drawbacks. For example, where shims are used, there is the potential that one or more shims may come loose at an inopportune time. Further, the use of shims requires that maintenance personnel maintain extremely close attention to the type and number of shims used during disassembly and reassembly.